The best and cheapest way is to get a old tin bread bin (I've used a roses tin but there plastic now) and drill holes in it to take coat hanger wire , pass the wire through and bend the ends to hold in place on top of ya wires place a bit chicken wire to hold the fish or fillet ,put a couple of bent wires to lift it out when cooked , all you need is a fire set in a barbecue with sticks a handful of oak chips or shavings in the bottom and the fish in the tin with lid on and sit on the fire for 5 mins depending on fish size.
5mins does for mackerel fillets also lightly oil the wire to prevent sticking and put skin down . Tip for mackerel fillets , salt for 1/2 hour then rinse and dry with kitchen paper , the smoke adheres better .
I've done chicken fillets ( which are great on a saled) and boiled eggs shelled and put in for 4 mins ( great in egg mayo )
Hope this helps and if you need some oak chips I have a sack full , ya welcome to experiment with some
Have fun , Paul